Magnetic hard-disk drives (“HDDs”) can store and retrieve large amounts of information. The information is commonly stored as a series of bits on a stack of thin-film magnetic disk platters, each of which is an aluminum alloy or glass substrate coated on each side with thin-film magnetic materials layers and one or more protective layers. A bit is identified as a flux transition. Linear bit density is measured as the number of flux transitions per unit length, and areal bit density is measured as the number of flux transitions per unit area. Typically, the higher the linear and areal bit densities, the lower the signal-to-noise ratio. Read-write heads, typically located on both sides of each platter, record and retrieve bits from circumferential tracks on the magnetic disks.
FIG. 1 shows a cross-section of a conventional magnetic disk that uses a laminated information layer 100. The laminated information layer 100 includes upper and lower magnetic layers 104a and b, with the upper layer 104a (CoCr PtB) containing the recorded information. The lower layer 104b (CoCr-alloy) is formed above an underlayer 108 (Cr-alloy), a seed layer 112 (Cr), and a supporting substrate 116. The parallel orientations of the moments 120a and 120b add constructively to provide a high effective magnetic moment for the laminated magnetic layer 100. Average grain diameters are now less than 10 nm. Laminated information layers are further described in U.S. Pat. Nos. 6,007,924; 6,610,424; and 6,677,051, each of which is incorporated herein by this reference.
The use of smaller grain sizes has a detrimental impact on the thermal stability of grain magnetization, particularly at high bit densities where the demagnetizing fields are significant. The equation which determines the stability of a recording medium against thermal fluctuations is KuV/kBT, where Ku is the magnetic anisotropic energy of the magnetic medium, V is the volume of a magnetic grain, kB is Boltzmann's constant, and T is the absolute temperature. Magnetic media having higher values for KuV/kBT are generally more stable against thermal fluctuations. When magnetic media have lower values and are therefore thermally unstable, increases in temperature can cause loss of stored information through the onset of the superparamagnetic effect. When a magnetic recording layer exhibits superparamagnetic behavior, the layer, in the remanent state (in the absence of an applied magnetic field), returns to its lowest energy state in which the magnetic domain states are randomly distributed. Flux transitions recorded in the layer are generally lost when the layer behaves superparamagnetically.
Attempts to control thermal instability typically attempt to increase the value of the numerator in the above equation, namely KuV. In one approach, a higher anisotropy material is used to provide a higher value for Ku while maintaining the grain volume at a low level to realize desired linear and areal densities. However, the increase in Ku is limited by the point where the coercivity Hc, which is approximately equal to Ku/Ms becomes too great to be written by a conventional recording head. As will be appreciated, the “coercivity” of a magnetic material refers to the value of the magnetic field required to reduce the remanence magnetic flux to zero, i.e., the field required to erase a stored bit of information. In the other approach, the effective magnetic volume V of the magnetic grains is increased.
FIG. 2 shows a cross-section of a magnetic disk that provides a low remanence-squareness-thickness-product (“Mrt”) while maintaining a high magnetic volume V, thereby providing greater degrees of thermal stability. As will be appreciated, the Mrt is the product of the remanent magnetization Mr, the magnetic moment per unit volume of ferromagnetic material, and the thickness t of the magnetic layer. The disk employs a laminated information layer 200 formed above an underlayer 204 (Cr-alloy), a seed layer 208 (Cr), and supporting substrate 210. In the laminated information layer, the magnetic moments 212 and 216 in the upper (CoCr PtB) and lower ferromagnetic layers 220 and 224, respectively, are antiferromagnetically exchange coupled across a very thin (less than 10 Å thick) nonmagnetic spacer layer 228 (which is typically pure (undoped) ruthenium). The anti-parallel orientations of the moments 212 and 216 add destructively to provide a low net magnetic moment for the laminated magnetic layer 200. The thermal stability of the laminated layer 200 is, theoretically, substantially enhanced because the grains in the lower magnetic layer 224 are magnetically exchange coupled with the grains in the upper magnetic layer 220 and thus the physical volume of layers 220 and 224 add constructively to provide a higher value for V. Thus, the layers can contain very small diameter grains while theoretically maintaining good thermal stability. Anti-ferromagnetically exchange coupled on AFC media are further discussed in U.S. Pat. Nos. 6,602,612 and 6,280,813, each of which is incorporated herein by this reference.
The annular disk shape has complicated the ability to obtain further significant increases in bit density because of the existence of differing operating conditions in different parts of the disk. To obtain higher data rates, the rotation speeds of hard disks are increasing, with speeds of 10,000 to 15,000 rpm now being common. Due to the annular shape of disks, the lengths of the inner tracks (in the inner diameter (“ID”) disk region) are significantly less than the lengths of the outer tracks (in the outer diameter (“OD”) disk region), and therefore the track velocity in the ID region is less than the track velocity in the OD region. As shown in FIG. 3 for a given rpm and linear density, the recording frequency increases dramatically from the ID to the OD regions.
The disparate track velocities in the ID and OD disk regions together with the substantial uniformity in disk properties across the face of the disk cause the User Bit Density or UBD in the ID and OD regions to be subject to different limiting factors. To obtain adequate bit error rates at high recording frequencies, adequate signal-to-noise ratios have to be achieved. The bit error rates typically correlate to the Spectral Signal-to-Noise Ratio or SpSNR, which is defined as the ratio of the signal amplitude to the total noise at half of the highest recording density. As shown by the following equation, the total noise, Nt, is given by the relationship:Nt2=Ne2+Nm2 where Ne is the electronic noise from the recording head circuit and Nm is the medium noise. The electronic noise Ne increases with the recording frequency f as given by the equation:Ne2=(0.9 nV2)×fMHZIn FIG. 4, the electronic noise is calculated using the above equation and plotted versus the recording frequency (horizontal axis). As shown in FIG. 4 for a given linear density, Ne (vertical axis) increases from the ID to the OD regions. As shown in FIG. 5, reducing the grain volume by either decreasing the grain diameter or decreasing the remanence-squareness-thickness-product (“Mrt”) reduces the media noise. The desired SpSNR value could thus be realized by controlling the actual contributions of the electronic and media noise components in the total noise. As shown in FIG. 6, which considers a rotation of 10,000 rpm and maximum linear bit density of 600 KFCI, the medium noise dominates the total noise in the ID region while the electronic noise dominates the total noise in the OD region. Stated another way, in the ID region the SpSNR improves when a lower Mrt is used while in the OD region the SpSNR improves when a higher Mrt is used.
Increasing the Mrt from the inner diameter to the outer diameter also increases the coercivity from the inner diameter to the out diameter. As will be appreciated, the coercivity of a medium is determined by the process conditions (e.g., substrate temperature and underlayer, seed layer, and intermediate layer structures and the magnetic layer alloy composition). As the conditions are uniformly applied to the disk, the coercivity cannot normally be controlled independent of the Mrt. A higher coercivity of the outer diameter region decreases the overwrite performance. Due to higher linear velocity, the head flies higher at the outer disk diameter and therefore the outer diameter region of the disk has usually lower overwrite performance than the inner diameter region and further increasing the coercivity in the outer diameter region therefore degrades the overall performance. As will be appreciated, overwrite refers to the ability of the recording head to erase the previously written information and write new information.
There is thus a need to provide magnetic media having a lower Mrt in the ID region and a higher Mrt in the OD region while at least substantially minimizing any increase in coercivity at the outer diameter of the media.